(1) Field of the Invention
This invention relates in general to semiconductor device processing, and more particularly to a method for improving the adhesion between a layer of a spin-on-glass and a layer of another material.
(2) Description of the Prior Art
In general, integrated circuits are built by depositing a series of layers one on top of another. After the deposition of each layer, and prior to the deposition of the next one, the layer is subjected to an etching procedure which shapes it into a pattern of lines and distinct areas. As a result, what started out as a completely planar surface becomes increasingly more irregular as successive layers of material partially cover the surface. Thus, it becomes necessary to include in the process of making an integrated circuit some means for returning the surface to a planar condition. If this is not done, problems soon arise with, for example, the alignment of successive masks that are used to define the patterns into which the different layers will be etched.
One way of achieving planarization is to deposit a layer that is initially in the liquid state. Such a layer will seek its own level rather than contouring the surface onto which it is deposited--as is the case with layers that are deposited directly in solid form. It then remains to convert the liquid layer into a solid one (with properties that are compatible with the intergrated circuit as a whole, including any subsequent manufacturing steps). Assuming the newly created solid layer retains the same surface geometry as the liquid layer from which it derived, planarization of the surface will have been achieved. One class of materials that can start out as liquids and then be converted to suitable solids are the siloxanes.
Siloxanes are a class of compounds containing silicon, oxygen, and hydrocarbon radicals such as CH.sub.3 or C.sub.2 H.sub.5. Spin-on glasses are created by dissolving a siloxane in a suitable solvent, such as methyl alcohol, adjusting the concentration of the solution so as to obtain the right viscosity, and then applying it to the surface of, for example, a semiconductor wafer so as to form a layer of a predetermined thickness. Thickness is controlled by dripping a measured amount of the solution onto the surface of a spinning wafer in the same way that photoresist is applied. The siloxane solution is then allowed to dry, following which it is subjected to heat treatment (typically 250.degree. C. for approximately 5 minutes) to drive out the solvent, followed by curing in an oxygen atmosphere at about 420.degree. C. During curing the silicon in the siloxane is converted to SiO.sub.2 while the other components are converted to volatile byproducts, such as H.sub.2 O and CO.sub.2. The resulting layer is referred to as cured spin-on-glass (SOG).
Once the SOG has been cured it may be etched back to some desired thickness. After the optional etch back procedure, the next step in the manufacture of the integrated circuit would be to deposit an additional layer of insulating or conductive material onto the surface of the SOG. If this is done immediately, there should be no problem relative to the adhesion of the newly deposited film to the surface of the cured SOG. However, we have observed that if a relatively short time of only about three hours (or more) is allowed to elapse between these two steps, an adhesion problem is likely to arise. This adhesion problem manifests itself as a series of bubbles or blisters that appear at the interface between the surface of the cured SOG and the layer immediately above it.
One way of dealing with this problem that has been described in the prior art has been to etch away the cured SOG so that the original surface over which the film of siloxane solution was spread is exposed once again. Such a procedure is described by, for example, A. Malazgirt et al in U.S. Pat. No. 4,986,878 Jan. 1991. This is not an adequate solution to the problem since SOG still remains in the valleys between the higher areas of the original surface. If etching were allowed to continue beyond the point at which the first of the original surfaces were once again exposed, material would be removed that needed to stay behind. Furthermore, stopping the etch-back procedure at exactly the right point presents a formidable control problem. An additional, though less serious, objection to this approach is that the layer of cured SOG that has been removed must now be replaced by a layer of some other material.
Heat treatment of a cured SOG has been described in the prior art but the purpose of such heat treatment as well as the manner in which the heat is applied and the timing within the overall integrated circuit manufacturing process at which these heat treatments occur are all different from the present invention.
In general, use of heat treatment for cured SOGs has been motivated by a need to solve the inversion problem. The inversion problem is a consequence of the fact that certain small metallic ions, particularly sodium ions, are known to have the capability to diffuse rapidly through a SOG because of the latter's relatively porous structure. When such ions end up near a semiconductor surface that is in contact with the SOG, they electrostatically attract negative carriers within the semiconductor and increase their concentration near the SOG-semiconductor interface. This local concentration of negative carriers is termed an inversion layer. Such a layer is highly undesirable since, at worst, it can cause a metal-oxide-semiconductor (MOS) device to be permanently ON or OFF and, at a minimum, it will change the operating characteristics of the device in an unpredictable manner.
An example of heat treatment of a SOG that has been described in the prior art is that given by Lin et al in U.S. Pat. 07/825,371 Jan. 1992. The method of that invention requires that the heat treatment be performed in an atmosphere of nitrogen gas. Quite a different method for dealing with the inversion problem is that described by Liu in U.S. Pat. 5,254,497 Oct. 1993. In this approach, the cured SOG, as well as any subsequently deposited metallurgy, is irradiated with ultra-violet light.
An important objective of this invention has been to come up with an effective solution to the problem of poor adhesion to SOGs that have been exposed to atmosphere for a period of about a few hours, or more, prior to the deposition of another layer.
A further objective of the present invention has been to provide a solution that does not require the removal of any part of the SOG layer itself. Such a solution has been achieved and takes the form of providing an effective heat treatment at the correct point in the overall process.